1. Field of the Invention
The present invention relates to an engine output control system for controlling an engine output in accordance with an amount of acceleration pedal operation, more specifically to an control system provided with both an electrical control mechanism and a mechanical control mechanism which are selectively employed for controlling the engine output.
2. Description of the Prior Art
Japanese Patent Public Disclosure No. 51-138235 laid open to the public in 1976 discloses an engine output control system for controlling an opening of a throttle valve in accordance with an acceleration pedal operation so that the engine output is controlled in response to an engine operating condition.
Japanese Patent Public Disclosure No. 59-12742 laid open to the public in 1984 discloses an engine output control system in which an mechanical control system for mechanically controlling the opening of the throttle valve is actuated to provide a minimum opening of the throttle valve when an electrical control system for electrically controlling the opening of the throttle valve is out of order so that a certain engine output can be obtained. When the electrical control mechanism is restored, the engine output control is switched to the electrical control again.
It should however be noted that a torque shock may be produced when the electrical control is switched to the mechanical control because the electrical control is different from the mechanical control in control property.
Meanwhile, the control system as disclosed in the Japanese Patent Public Disclosure No. 51-12742 is usually provided with a magnetic clutch disposed between a drive motor for the throttle valve and the throttle valve for switching the engine output between the electrical control and the mechanical control wherein the electrical control is exerted when the magnetic clutch is engaged and the mechanical control is exerted when the magnetic clutch is disengaged. It is disadvantageous in that there is a certain delay time before an actual engagement force is produced after the magnetic clutch is turned on. As a result, a slippage is produced in the magnetic clutch in the case where the motor is actuated to move the throttle valve. This means that the delay time in the clutch causes a wear of the clutch and deteriorates a controllability of the throttle valve.